The instant invention relates to short arc lamps, and more specifically to a short arc lamp having an improved housing structure which simplifies manufacturing and reduces cost, while also improving structural integrity and thermal performance.
Short arc inert gas lamps are well known in the prior art for use in applications requiring high intensity light, such as for example, in spectroscopy, or in other fiber optics illumination devices, such as endoscopes for the medical industry. Short arc lamps generally comprise a sealed chamber containing an inert gas, such as xenon, pressurized to several atmospheres, and an opposed anode and cathode defining an arc gap. The application of electricity to the anode and cathode cause an arc which glows brightly in the inert gas. A reflective surface within the chamber reflects light outwardly through a window. While the general configuration of these lamps is well known there are many different variations in the specific implementation. The variations are due to two significant issues that are paramount in the design and construction of such a lamp. The first issue is structural integrity of the housing to maintain the inert gas at elevated pressures and the second issue is heat transfer. Short arc lamps of this type operate at extremely high temperatures. Accordingly, there are many design issues in attempting to maintain structural integrity and also dissipate heat from the overall housing.
Throughout the prior art there have been many attempts to modify and improve both the structural integrity of the housing and to improve the thermal performance. In this regard, the U.S. patents to McRae et al U.S. Pat. No. 3,731,133; Roberts et al, U.S. Pat. No. 4,599,540; Roberts et al, U.S. Pat. No. 4,633,128; Roberts, U.S. Pat. No. 5,399,931; Takahashi et al U.S. Pat. No. 5,789,863; Sugitani et al, U.S. Pat. No. 5,903,088; Tanaka et al, U.S. Pat. No. 6,281,629 and Kiss et al U.S. Pat. No. 6,285,131 represent the closest art to the subject invention of which the Applicant""s are aware.
Each of the patents listed hereinabove describes a short arc lamp comprising a ceramic body structure having a concave reflective surface, a conductive base structure supporting the anode, and a conductive window assembly supporting the cathode. The U.S. patent to McRae et al U.S. Pat. No. 3,731,133 is directed to a short arc lamp wherein the reflector surface of the ceramic body is metalized to provide the reflective surface. The U.S. patent to Roberts et al, U.S. Pat. No. 4,599,540 discloses a short arc lamp wherein the reflector surface of the ceramic body is formed by pressing the ceramic body, when hot, with an unpolished mandrel for greater accuracy in formation of the reflective surface configuration. The U.S. patent to Roberts et al, U.S. Pat. No. 4,633,128 concerns another embodiment of a short arc lamp wherein the ceramic reflector body is provided with a convex space behind the reflector surface so that the reflecting wall is relatively thin near the focal point of the lamp. A copper sleeve is attached to the reflecting wall within the convex space to conduct heat away from the reflecting wall. The U.S. Patent to Roberts, U.S. Pat. No. 5,399,931 is a further improvement to the Roberts ""128 patent wherein a copper heat transfer pad is brazed to a base assembly and to an exterior ring such that heat is more efficiently transferred to the outside surfaces of the lamp. The U.S. Patent to Takahashi et al U.S. Pat. No. 5,789,863 is directed to a short arc lamp having a single cantilevered cathode support arm which is intended to reduce thermal influences in positioning of the tip of the cathode. The U.S. Patent to Sugitani et al, U.S. Pat. No. 5,903,088 discloses a short arc lamp wherein a gap is provided between a cathode support ring and an exterior conductive ring, and another gap is formed between a window support ring and the cathode support ring. The U.S. Patent to Tanaka et al, U.S. Pat. No. 6,281,629 concerns a short arc lamp structure wherein a heat transfer plate is positioned between the base and the ceramic body. The heat transfer plate has a higher thermal conductivity than the base. Finally, the U.S. Patent to Kiss et al U.S. Pat. No. 6,285,131 is directed to an arc lamp wherein the cathode suspension system is stamped from a sheet of Kovar(copyright) (Kovar(copyright) is a registered trademark of Westinghouse Electric) material and then brazed to an annular support ring.
While each of the above-noted devices is suitable and effective for the intended purpose, they are generally complex in construction and difficult to fabricate, and thus expensive to manufacture. There is thus a need in the art for an improved short arc lamp that concurrently simplifies construction while improving structural integrity and thermal performance.
The instant invention provides such a novel short arc lamp having an improved housing structure which simplifies manufacturing and reduces cost, while also improving structural integrity and thermal performance.
The improved housing structure for a short arc lamp includes a ceramic body having a concave reflective surface formed in an upper end thereof, a base adapted to receive the base end of the ceramic body in abutting relation, and a window frame assembly positioned in abutting concentric relation with the upper end of the ceramic body.
The ceramic body comprises a cylindrical mass of alumina having a first end in which a concave reflector surface is formed. The reflector surface has an axis of rotation and a focal region defined along the axis of rotation.
The base is integrally formed with a shoulder ring adapted to receive and seal the base end of the ceramic body. Integrated formation of the shoulder ring with the base has been found to provide a significant improvement in manufacturing, as the base, ceramic body, anode, exhaust tubulation and window frame ring can be easily assembled and brazed in a single brazing operation. In particular, the base is preferably formed from an iron alloy and more preferably formed from an alloy of iron, nickel and cobalt using a metal injection molding (MIM) metallurgical forming process. MIM provides the ability to mold complex geometries in a solid part that would not be feasible in conventional milling operations or may not be cost effective.
The window frame structure is integrally formed to include an annular flange having a substantially U-shaped cross-section and three circumferentially spaced cathode support arms extending radially inwardly therefrom. The cathode support arms further include an integrally formed cathode mounting ring at the terminal intersection thereof. The window frame structure is also preferably formed using MIM forming techniques so that the window frame and cathode support arms are formed as a single unitary structure. Forming the cathode support arms as an integral portion of the frame eliminates at least one brazing step from the prior art techniques and further eliminates the separate manufacturing step of forming the cathode support arms. In the prior art, the cathode support arms were formed separately and then brazed together with the annular flange of the window frame. Axial alignment and position of the cathode support arms was difficult and time consuming in the manufacturing process. Integrally forming the annular flange, cathode support arms and the cathode mounting ring improves the accuracy of axial alignment of the cathode. In the assembly process, a sapphire window and a cathode are assembled together with the window frame structure, and brazed together in a single process to provide a completed window frame sub-assembly.
As indicated above, the novel changes in construction of the components significantly simplifies the assembly process. In the preferred method of assembly, the anode, exhaust tubulation, ceramic body and window frame ring are assembled with the base and simultaneously brazed together in a single operation to form a body sub-assembly.
The window frame sub-assembly is then joined to the window frame ring of the body sub-assembly to complete the assembly.
The present short arc lamp is also optimized for thermal performance in another alternative embodiment. In this alternative embodiment, the ceramic body is formed from beryllia (beryllium oxide) which has superior thermal transfer characteristics. The alternative embodiment is further provided with a coating which helps keep infra-red (IR) light energy from escaping from the window of the lamp. In one instance, the coating is an IR reflective coating placed on the window of the lamp to reflect IR light energy back into the lamp where it can be conducted outwardly through the base. In another instance, the reflector surface is provided with a dichroic coating which reflects visible light, while allowing IR energy to pass through. Accordingly, the IR energy passes through to the ceramic body and is transferred outwardly through the base. In yet another instance, the wavelength selective coatings are applied to both the reflector surface and the window.
Accordingly, among the objects of the instant invention are: the provision of an improved short arc lamp having a simplified construction of the window support, the base assembly and the body; the provision of a window support of a single piece construction that supports the cathode, supports the window, provides thermal conduction of the cathode and provides electrical conduction to the cathode; the provision of a base assembly that can be sealed to the anode, the reflector body, and to the exhaust tubulation in a single brazing operation; the provision of a base that increases the surface area of the base without altering the current footprint and that also increases thermal conduction to the external surface; the provision of an improved short arc lamp wherein the ceramic reflector body is fabricated from beryllium oxide to improve thermal conduction of heat from the lamp to the external surfaces; and the provision of an improved short arc lamp wherein the window is provided with an infra-red coating to reflect IR energy back into the lamp, or the reflector surface is provided with a dichroic IR pass-through coating, or both.